Buildings, towers and similarly heavy structures commonly are built on and around a steel framework. A primary element of the steel framework is the joint connections of the beams to the columns. An improved structural joint connection is disclosed in U.S. Pat. No. 5,660,017. However, advanced stress analysis techniques and a study of building collapse mechanisms following seismic and blast events (i.e., terrorist bombings) have resulted in the present improvements.
Further, consideration of the conventional building erection tasks and methodologies employed when erecting a building or constructing components for such a steel frame building (as well as the on-site erection of the buildings themselves), with joint connections including gusset plates (or side plates) spanning a column and receiving an end portion of a beam therebetween, has also resulted in the recognition of several inefficiencies or problem areas. Hereinafter, the gusset plates (or side plates) are referred to with either term (or with both terms) as one term has to do with the function of the plates as reinforcement or strengthening to a beam-to-column joint, and the other term has to do with the location of the plates on the sides of the columns and beams. Moreover, as a result of the deficiencies of the conventional technologies, construction costs and material costs for a steel frame building structure of conventional construction are significantly higher than necessary. That is, the current technology teaches a beam (or beams)-to-column joint structure for joining one or more beams in a supporting relationship to a column, with each joint structure including a pair of gusset plates (or side plates) spaced apart and spanning the column, and sandwiching between them the column and an end portion of a connecting beam or beams. The gusset plates or side plates extend outwardly from the column along the sides of the beam(s). Of course, as taught in U.S. Pat. No. 5,660,017, the gusset plates may extend in both directions from a column so that they extend across the column, and connect two beams together, in a supporting relationship to the interposed column.
Conventionally, in preparation for erection of such a steel frame building, column structures are shop fabricated, adding the gusset plates or side plates to column sections for one or more floors of the building to be erected at a building site. Between the gusset plates or side plates, an end portion (or stub) of connecting beam is secured into each joint assembly, as by welding. Additional components of the joint assembly are generally added to the columns at this time also, such as welded in vertical shear plates and welded in horizontal continuity plates or shear plates, which improve the strength and stiffness of the joint assemblies. These additional components also facilitate load transfer between the principal components of the joint assembly.
Such column structures or assemblies are then shipped to a construction site where the column assemblies for one or more of the lower floors of the building are properly aligned to one another, and are set in the building foundation. With the column assemblies so set and aligned, the conventional practice is then to connect each two aligning stub beams of adjacent column assemblies with a so-called link beam. This link beam is simply an elongate steel beam section generally matching the two stub beams to be connected, and of the proper length to fit between these stub beams with a proper welding root gap. The link beam is then welded in the field (i.e., at the construction site) at each of its ends to one of the aligned stub beams of the connected joint assemblies. Understandably, fitting such link beams into place, and making the field welds at each end of such link beams, which are necessary to structurally join the beam stubs and link beam, is a labor intensive and expensive process. The field welding necessary for this joining of beam stubs to link beams will require multiple passes, and it is to be understood that the beam stubs and link beam may be 30 inches to 42 inches, or more in the vertical dimension and 10 inches to 14 inches or more in the horizontal dimension, so each field weld (required to connect the web of a beam stub to the web of a link beam, and to connect the flanges of a beam stub to the flanges of a ling beam) is a big and labor intensive job to be done in the field. Further, these welding jobs must be performed at heights above the ground that make working and welding a somewhat risky operation. Depending on the design height of the building, construction of successive floors or groups of floors proceeds upwardly atop of the framework for the lower floors. Consequently, as the building grows upwardly, the heights at which such link-beam-to-beam-stub welds must be done grows progressively also.
Moreover, during the last several years, there has been considerable additional concern as to how to improve the beam-to-column, and beam-to-beam joint connections of a steel frame building so they will better withstand explosions, blasts and the like as well as other related extraordinary load phenomena. Of particular concern is the prevention of progressive collapse of a building if there are one or more column failures due to terrorist bomb blast, vehicular and/or debris impact, structural fire, or any other impact and/or heat-induced damaging condition.
Column failures due to explosions, severe impact and/or sustained fire, have led to progressive collapse of entire buildings. An example of such progressive collapse occurred in the bombing of the A. P. Murrah Federal Building in Oklahoma City in 1995 and in the aerial attack on the World Trade Center towers in 2001.
Following the 1994, Northridge, Calif. earthquake, in addition to the invention set forth in U.S. Pat. No. 5,660,017, a number of other alternatives to resist joint connection failure, were suggested or adopted for use in steel construction design for improved seismic performance. For example, the reduced beam section (RBS), or “dog bone” joint connection has been proposed, in which the beam flanges are narrowed near the joint connection. This alternative design reduces the plastic moment capacity of the beam allowing inelastic hinge formation in the beam to occur at the reduced section of the beam. This inelastic hinge connection is thought to relieve some of the stress in the joint connection between the beam and the column. An example is seen in U.S. Pat. No. 5,595,040, for Beam-to-Column Connection, which illustrates such “dog bone” connections. But, because the plastic moment capacity of the beam is reduced due to the narrowing of the beam flanges, the moment load which can be sustained by the beam is also substantially reduced.
Another alternative is illustrated by U.S. Pat. No. 6,237,303, in which slots and holes are provided in the web of one or both of the column and the beam, in the vicinity of the joint connection, in order to provide improved stress and strain distribution in the vicinity of the joint connection. Other post-Northridge joint connections are also identified in FEMA 350—Recommended Seismic Design Criteria for New Steel Moment Frame Building, published by the Federal Emergency Management Agency in 2000. All such post-Northridge joint connections have reportedly demonstrated their ability to achieve the required inelastic rotational capacity to survive a severe earthquake.
However, one important consideration to be noted in contrast to the present invention is that none of these alternative joint connections provide independent beam-to-beam structural continuity across a column; such continuity being capable of independently carrying gravity loads under a “double-span” condition resulting from a column being suddenly or violently removed by, for example, explosion, blast, impact or other means, regardless of the damaged condition of the column. Additionally none of these alternatives, except the gusset plates used as taught in U.S. Pat. No. 5,660,017, provide any significant torsion capacity or significant resistance to lateral bending to resist direct explosive air blast impingement and severe impact loads. Torsion demands for the joint are created because while the top flanges of the beams are typically rigidly attached to the floor system of a building against relative lateral movement, the bottom flange of the beam is free to twist when subjected to, for example, direct lateral blast impingement loads caused by a terrorist attack. A structure according to this invention will sustain such “double-span” conditions as well as demands from severe torsion loads; while also providing advantages in savings of material, weight, and labor. Indeed, there are no additional and discrete load paths across the column in the event of column failure or joint connection failure or both.